WO2012050775A1

WO2012050775A1 - Variable sensitivity interferometer systems

Info

Publication number: WO2012050775A1
Application number: PCT/US2011/052610
Authority: WO
Inventors: Edward Tapanes
Original assignee: Fiber Sensys, Inc.
Priority date: 2010-10-14
Filing date: 2011-09-21
Publication date: 2012-04-19
Also published as: CA2813869A1; IL225625A0; EP2627966B1; IL225625A; CN103261835B; KR20130090414A; US20120224182A1; US8873064B2; WO2012050774A1; AU2011314185A1; RU2013119231A; US9400167B2; US20150062588A1; KR101522318B1; CA2813869C; KR20130085037A; EP2627966A1; EP2627966A4; US20130208283A1; CN103261835A

Abstract

Variable Sensitivity optical sensors can have a respective actual sensitivity of one or more portions of the sensor corresponding, at least in part, to a selected environment of each respective sensor portion. Some disclosed sensors have a plurality of optical conduits extending longitudinally of the sensors. At least one of the optical conduits can have at least one longitudinally extending segment having one or more optical and/or mechanical properties that differs from the optical properties of an adjacent longitudinally extending segment, providing the conduit with longitudinally varying signal propagation characteristics. An optical sensor having such optical conduits can exhibit a longitudinally varying actual sensitivity. Nonetheless, such a sensor can exhibit a substantially constant apparent sensitivity, e.g., when each respective portion of the sensor exhibits an actual sensitivity corresponding to a selected environment. Innovative sensors can provide a low- incidence of false or nuisance alarms, accurate position and magnitude information, and other advantages.

Description

VARIABLE SENSITIVITY INTERFEROMETER SYSTEMS

Inventor: Edward Tapanes

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Patent Application Serial Number 61/393,298 and U.S. Provisional Patent Application Serial Number 61/393,321 (referred to hereinafter as the "System Patent Application"), both filed October 14, 2010, the contents of which are hereby incorporated by reference as if recited in full herein for all purposes.

BACKGROUND

The innovations disclosed herein pertain to interferometer systems, and more particularly, but not exclusively, to fiber-optic interferometer systems having variable- sensitivity sensors. Some innovative interferometer systems are configured to detect and/or locate disturbances (e.g., a disturbance to a secure perimeter, such as a "tap" on a fence, a leak from a pipeline, a change in structural integrity of a bridge, a disturbance to a communication line, a change in operation of a conveyor belt, an impact on a surface or acoustical noise, among others). In some instances, such systems can be configured to detect and/or locate over distances up to, for example, about 65 kilometers (km) with one passive sensor, and up to, for example, about 130 km with first and second passive sensors extending in opposite directions.

Most optical conduits (e.g., optical fibers) have substantially uniform (e.g., unvarying) properties along their lengths, making such optical conduits suitable for a wide variety of applications (e.g., communications, interferometers) that demand homogeneous properties. This homogeneity is a result of, among many factors, modern high-quality manufacturing processes for optical fibers, coatings on the optical fibers and various protective sheaths of the cable in which the fibers are typically encased. When used as a sensor configured to detect a disturbance, such longitudinally homogeneous optical conduits typically respond to a given perturbation in a uniform manner, regardless of where the perturbation is applied along the sensor's length.

In many applications, different portions of a given sensor can be exposed to respective different environments. For example, a portion of a sensor can be positioned, for example, under water, another portion can be positioned underground and yet another portion can be positioned above-ground (e.g., exposed to the atmosphere). In such an application, known sensors can respond to a given disturbance differently depending, for example, on the environment and which portion of the sensor is perturbed. Therefore, with known sensors having homogeneous sensitivity, it can be difficult to discern one or more characteristics (e.g., amplitude, position, etc.) of any particular disturbance, particularly if the environmental surroundings vary along the sensor's length.

Accordingly, known fiber-optic sensors can be prone to initiating "false" or "nuisance" alarms. Although some environmental effects can be filtered mathematically to reduce a rate of false and nuisance alarms, such algorithms can be computationally intensive and can lead to intermittent operation. Moreover, such mathematical filtering may not satisfactorily reduce the occurrence of false or nuisance alarms.

Accordingly, there remains a need for sensors, e.g., passive fiber-optic sensors, configured to extend through more than one environment while responding similarly to a given disturbance regardless of the environment. Other needs relating to sensing systems are also unmet.

SUMMARY

Innovative optical sensors and related interferometer systems addressing one or more of the above-identified and other needs are disclosed. Some embodiments of such innovations include a sensor having an actual sensitivity that varies along its length.

A sensor having substantially constant properties along its length typically has a substantially constant actual sensitivity along its length. A given disturbance can be conveyed to a sensor through one environment differently than through another environment, making a sensor's response to such a disturbance appear to be

environmentally dependent. Moreover, sensors with longitudinally uniform properties exhibit an apparent sensitivity in one portion exposed to a given environment that differs from an apparent sensitivity exhibited by another portion of the sensor positioned in another environment. As used herein, "actual sensitivity" means a measure of a sensor's response to a given disturbance in a selected reference environment. As used herein, "apparent sensitivity" means a measure of a sensor's response to a given disturbance in an arbitrary environment. For example, a singlemode interferometer buried in the ground might produce 10 interference fringes in response to a given physical disturbance. The same interferometer (or a portion thereof) positioned above-ground might produce 500 interference fringes in response to a similar disturbance.

In contrast to a sensor having longitudinally homogeneous optical properties, a sensor having longitudinally varying optical properties, and a corresponding

longitudinally varying actual sensitivity, can provide a substantially constant apparent sensitivity when the sensor extends through a variety of environments. Innovative optical sensors are disclosed in which the respective actual sensitivity of one or more portions of the sensor correspond, at least in part, to a selected environment of the respective sensor portions.

For example, some disclosed sensors have a plurality of optical conduits extending longitudinally of the sensors. At least one of the optical conduits can have at least one longitudinally extending segment having one or more optical and/or mechanical properties (e.g., birefringence, fiber coating, sheaths, etc.) that differ from the optical properties of an adjacent longitudinally extending segment, thus providing the conduit with longitudinally varying signal propagation characteristics. An optical sensor having one or more such optical conduits can exhibit a longitudinally varying actual sensitivity. Nonetheless, such a sensor can exhibit a substantially constant apparent sensitivity, such as when the sensor extends through a plurality of environments (e.g., as a pipeline can), particularly when each respective portion of the sensor exhibits an actual sensitivity corresponding to a selected environment. Such an innovative sensor can provide a low- incidence of false or nuisance alarms, as well as accurate position and magnitude information.

Some innovative systems comprise a method for detecting a disturbance with a sensor having a position-dependent actual sensitivity. With some embodiments of such innovative systems, a position of a disturbance can be determined, and, in some instances, a magnitude of such a disturbance can also be determined when the sensor spans a variety of environments (e.g., above ground, below ground, underwater and through open atmosphere). Some disclosed sensors are passively terminated and are configured to extend, for example, up to and even more than about 50 km away from active components. Some disclosed systems have two such sensors extending from the active components in opposite directions relative to each other, providing a disturbance-detection capability over large distances, for example, up to about 100-130 km.

The foregoing and other features and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings show aspects of the innovative systems disclosed herein, unless specifically identified as showing a known feature from the prior art.

FIG. 1 shows aspects of an innovative interferometer of the type disclosed herein.

FIG. 2 shows aspects of another innovative interferometer system of the type disclosed herein.

FIG. 3 shows a schematic illustration of a commercially available Mach Zehnder interferometer configured to use counter-propagating optical signals having actively matched polarization states.

FIG. 4 shows aspects of a sensor configured to provide an actual sensitivity that varies with longitudinal position; a portion of the sensor is shown in an enlarged view.

FIG. 5A shows a cross-sectional view taken along line 5A-5A in FIG. 4. FIG. 5B shows the cross-sectional view in FIG. 5A with several different pairings of optical conduits identified. Each unique combination provides the optical cable with a corresponding unique actual sensitivity. FIG. 6 shows a cross-sectional view taken along line 6-6 in FIG. 4.

FIG. 7 shows a sensor of the type disclosed herein extending among various environments, as when installed, for example, to monitor a pipeline.

DETAILED DESCRIPTION

Various principles related to optical conduits and interferometer systems are described herein by way of reference to exemplary embodiments. One or more of the disclosed principles can be incorporated in various configurations to achieve one or more performance characteristics. Disclosed embodiments of optical conduits and

interferometer systems relating to perimeter security applications are merely examples used to illustrate one or more of the innovative principles described herein. Some embodiments may be equally applicable to use in many other applications, such as, for example, detecting a leak in a pipeline, detecting a failure in a structure, detecting a disturbance to a ground surface, detecting a change in operation of a conveyor, etc.

Some innovative optical conduits disclosed herein can be combined with known interferometer configurations to provide levels of performance that heretofore have been unachievable. Examples of such innovative combinations are described below.

Overview of Interferometer Systems

Interferometer systems as disclosed herein can detect a disturbance to a sensor portion by comparing a phase shift between observed first and second optical signals that have travelled through a first (e.g., a "reference") optical conduit and a second (e.g., a "sensor") optical conduit. In systems disclosed herein, one or more optical and/or mechanical properties differ between the first optical conduit and the second optical conduit. For example, the innovative interferometer 100 shown in FIG. 1 is configured as a hybrid Michelson / Mach-Zehnder interferometer having an active portion 132a, as disclosed in the System Patent Application identified above, and a passive portion 130a. The interferometer 100a shown in FIG. 2 is also configured as a hybrid Michelson / Mach-Zehnder interferometer having an active portion 132b that includes a polarization scrambler, and a passive portion 130b. As shown in FIGS. 1 and 2, the systems 100, 100a include respective passive optical sensor portions 130a, 130b extending away from the respective active portions 132a, 132b toward a distal end 118.

FIG. 3 shows an interferometer system having overlapping first and second Mach- Zehnder interferometers configured to convey counter propagating optical signals.

A disturbance to one or both of the optical conduits 114a, 114b (or 114a', 114b', or the sensor portion of the overlapping Mach-Zehnder interferometers shown in FIG. 3) can modify each respective optical signal conveyed through the disturbed conduit. By observing such a modified signal, the existence of such a disturbance can be detected, and, in some instances, the position and magnitude of the disturbance can be identified.

OVERVIEW OF OPTICAL SENSORS

In some instances, the first and second optical conduits 114a, 114b (FIG. 1), 114a', 114b' (FIG. 2) can have similar optical and/or mechanical properties and similar lengths. In some embodiments, the reference and sensor optical conduits are physically separate conduits positioned adjacent each other in a "bundle" (also referred to as a "cable"). For example, a conventional fiber optic bundle can include several individual optical fibers (e.g., single-mode fibers) shrouded by an outer sheath(s). One of the individual optical fibers can define the sensor conduit (e.g., 114a) and another of the individual optical fibers can define the reference conduit (e.g., 114b). Yet another of the individual optical fibers can define a return conduit, such as in a passively terminated sensor as disclosed in System Patent Application identified above. Respective fibers defining the sensor, reference and return conduits can be positioned within and shrouded by the common outer sheath(s). Although such optical fibers are usually positioned relatively close to each other (e.g., within several millimeters of each other), a load or other force that alters the optical phase of the signals in the individual optical conduits (e.g., an impact or perturbation) applied to the outer sheath will be transmitted to each of the individual fibers slightly differently. Moreover, each of the individual fibers can respond (e.g., deform or momentarily have its refractive index changed) to identical loads somewhat differently. Thus, in practice, a disturbance to the cable generally will perturb the reference and the signal conduits 114a, 114b differently.

Since physical responses typically differ between the "sensor" conduit and the "reference" conduit, light travelling through the "sensor" conduit can arrive at a terminal end of the sensor conduit (FIGS. 1, 2 and 3) at a slightly different time, and possibly with a different polarization state, than light travelling through the "reference" conduit. Thus, optical signals observed at each respective terminal end will usually be out of phase from each other by some amount. When either or both of the sensor and reference conduits has been disturbed, the relative phase of the optical signals observed at each respective terminal end will tend to shift from the nominal level from the undisturbed conduits. By comparing a delay between the first of the optical signals and the second of the optical signals (e.g., an observed phase-shift between the signals), and accounting for

characteristics of the interferometer components (e.g., lengths of optical conduits, speed of light through the conduits, optical wavelength), the magnitude and position of a disturbance can be determined using a system as shown in FIGS. 1, 2 and 3.

Although many factors can cause an observed phase shift between signals conveyed through the first and second optical conduits, a nominal, or baseline, phase shift between observed signals of undisturbed reference and sensor conduits can be

determined. Thus, one can infer that a sensor cable (e.g., a bundle having a sensor conduit and a reference conduit) has been disturbed when a sufficiently large (or a threshold) deviation from a baseline phase shift is observed. In addition, observing such a phase-shift at more than one location through the optical path, combined with characteristics of the sensor cable (e.g., its length, the speed at which light travels through each of the optical conduits, optical wavelength), a location of the disturbance can be inferred.

As noted above, in some embodiments, a third, insensitive conduit can be positioned adjacent one or both of the sensor conduit (e.g., conduit 114a) and the reference conduit (e.g., conduit 114b). For example, an optical cable can have a plurality of optical conduits within a common sheath(s), as described above, and shown in FIG. 5A.

ENVIRONMENTAL DEPENDENCY OF OPTICAL SENSOR RESPONSE

As described above, a disturbance (e.g., an impact or perturbation) applied to the outer sheath of a cable typically will be transmitted to each of the individual optical conduits slightly differently. In addition, a disturbance to an environment surrounding or adjacent to a sensor cable 130a, 130b, 130c can be transmitted to the cable differently in one environment than in another environment. For example, a load transmitted to an underground sensor and arising from a given disturbance (e.g., someone digging a hole adjacent the underground sensor) typically differs from a load transmitted to an above- ground sensor arising from the same disturbance. Accordingly, a disturbance to each respective optical conduit in a sensor typically corresponds, at least in part, to the environment through which the sensor extends.

As a consequence, effects of such a disturbance on an optical signal propagating through the respective optical conduits also correspond, at least in part, to the

environment. It is believed that such effects at least partially contribute to observed variations in apparent sensitivity for a given sensor with longitudinally uniform properties extending through different environments.

SENSORS HAVING LONGITUDINALLY VARYING ACTUAL SENSITIVITY

As noted above, an optical sensor with a substantially constant actual sensitivity along its length can respond to disturbances in different environments differently, making it difficult to discern whether an observed event corresponds to a disturbance of the type intended to be sensed with systems of the types shown in FIGS. 1, 2 and 3. Accordingly, such an optical sensor can be prone to initiating "false" or "nuisance" alarms when the sensor extends through more than one environment. Although some false or nuisance alarms can be filtered mathematically, such algorithms can be computationally intensive and can lead to intermittent operation, without satisfactorily reducing the occurrence of false or nuisance alarms. As explained above, differing physical responses between a selected pair of optical conduits (e.g., the conduits 114a, 114b) can significantly affect an optical signal travelling through the respective conduits differently. An observed difference between such optical signals can be used to detect a disturbance to one or both of the conduits. As used herein, a "reference-sensor pair" means a selected pair of optical conduits (e.g., 114a, 114b) configured to convey respective optical signals and to operatively couple to one or more components (e.g., 132a, 132b) configured to respond to one or both of the optical signals.

Fiber optic sensors having longitudinally varying sensitivity are now described. For example, a distance between selected optical conduits forming a reference-sensor pair of conduits, a construction of each in the pair of conduits, or both, can vary along the sensor's length. Other physical characteristics (e.g., length, birefringence, sheath construction, cable fill material, polarization) can also vary along the sensor's length and provide a longitudinally varying physical response between selected reference-sensor pairs. FIG. 4 schematically illustrates one example 230 of an optical sensor having an actual sensitivity that varies longitudinally. FIGS. 5 A and 5B show a six-bundle cable capable of providing at least five different actual sensitivities, as described more fully below.

In FIG. 4, the sensor 230 extends between a proximal end 231 configured to operatively couple to an active portion of an interferometer (e.g., an interferometer 100, 100a, 100b shown in FIGS. 1 through 3, respectively) and a distal end 232. As noted above regarding the sensors 130a, 130b, 130c, the sensor 230 can be passively terminated at or near its distal end 232. The illustrated sensor 230 includes four segments 233, 235, 237, 239, each being configured to provide a respective actual sensitivity to disturbances. In particular, a first segment 233 extends between the proximal end 231 and a first joint 234; a second segment 235 extends between the first joint 234 and a second joint 236; a third segment 237 extends between the second joint 236 and a third joint 238; and a fourth segment 239 extends between the third joint 238 and the distal end 232. As will now be described, each of the first, second, third and fourth segments of the sensor 230 can be operatively configured to provide a corresponding unique actual sensitivity.

Each of the illustrated segments 233, 235, 237, 239 has a substantially identical construction. As can be seen in FIG. 5 A, the segment 235 includes six optical conduits 241, 242, 243, 244, 246, 247, each extending longitudinally of the segment; two such longitudinally extending conduits 243, 244 are shown in FIG. 6. In each segment shown in FIG. 4, the optical conduits (e.g., optical bundles) are circumferentially spaced from each other (e.g., at about 60-degrees from each other) around a central, longitudinal axis of the cable. Four of the six conduits, i.e., conduits 243, 244, 246 and 247, include tight- buffered fibers and are arranged in opposing pairs relative to the central longitudinal axis. The other two conduits 241, 242 include loose-tube fibers and are positioned about 180- degrees from each other, each being positioned between two respective of the conduits 243, 244, 246 and 247 having tight-buffered fibers.

Each of the six optical conduits can include at least one single-mode optical conduit. As used herein, "tight-buffered fibers" means a group of longitudinally extending optical fibers that are tightly packed (or held) into an operative structure and surrounding by dry materials. The tight-buffer fibers typically have a 900 micron outer diameter. As used herein, "loose-tube fibers" means a group of longitudinally extending optical fibers that are free-floating in a viscous (e.g., Newtonian) fluid, a gel, or a non- Newtonian fluid inside a dedicated fiber housing within the cable. In some

configurations, such a housing can be a tube. Such a gel or fluid (e.g., Newtonian or non- Newtonian) can tend to dampen a disturbance to the loose-tube fibers. The loose-tube fibers typically have a 250 micron outer diameter. Each fiber housing encasing the loose- tube fibers can have a plurality of fibers within, such as, for example, 6 or 12 fibers. Tight-buffered fibers are typically more responsive to a disturbance than loose-tube fibers.

In FIG. 5, an outer sheath 251 extends longitudinally of each segment 233, 235,

237, 239. Interstitial spaces 250, 250a in each segment can be filled with a suitable strengthening, packing and/or protective material. In some instances, such a suitable fill includes fiber-reinforced plastic, Kevlar fibers, water absorbing fabrics/tapes or other materials. In some instances, the interstitial spaces are filled with a material that dampens disturbances to the sheath 251, and in other instances the interstitial spaces are filled with a material that conveys such disturbances with minimal losses.

Each segment 233, 235, 237, 239 has a respective reference-sensor pair of optical conduits. FIG. 5B shows several possible reference-sensor pairs from which the respective reference-sensor pair can be selected. For example, a reference-sensor pair for a given segment 233, 235, 237, 239 can be formed from: (1) opposing tight-buffered fibers 243, 244; (2) opposing loose-tube fibers 241, 242; (3) loose-tube fibers 241 and adjacent tight-buffered fibers 243; (4) adjacent tight-buffered fibers 243, 246; or (5) a pair of loose-tube fibers 241 or 242 within a common loose-tube housing. Since the actual sensitivity relates, at least in part, to a distance separating the respective reference-sensor pair and the construction of each in the pair of fibers, each segment 233, 235, 237, 239 can have a unique actual sensitivity even though each segment has a substantially identical overall construction. For example, segment 233 can have the first (1) reference-sensor pair, segment 235 can have the second (2) reference- sensor pair, segment 237 can have the third (3) reference-sensor pair, and segment 239 can have the fourth (4) reference-sensor pair. Each of these reference-sensor pairs can be expected to differ in response to a disturbance, but each segment has a substantially identical construction, as explained above.

In FIG. 6, a portion 240 of the sensor is shown in longitudinal cross-section. Adjacent segments 235, 237 having substantially identical construction are shown, although the reference-sensor pairs differ in each segment, as just described.

Nonetheless, the segment 235 can be operatively coupled to the adjacent segment 237 using a conventional optical joint (e.g., fusion splice, mechanical splice, butt splice, etc.) 245a, 245b configured to join longitudinally adjacent conduits (e.g., to join conduit 241 to conduit 243 and conduit 242 to conduit 244, respectively). Such a joint can be an optical fiber fusion splice or a mechanical coupling.

Operatively joined conduits can have different constructions. For example, conduit 241 can be a loose-tube optical fiber and conduit 243 can be a tight-buffered fiber. With such a construction, the portion 240 of the optical sensor 230 can achieve an actual sensitivity that varies longitudinally (e.g., from segment 235 to segment 237). Respective segments joined (e.g., in end-to-end abutment) as just described can provide a sensor 230 having an actual sensitivity that varies longitudinally, despite that the configuration of each segment can be substantially identical to each other. Nonetheless, each segment can have a unique configuration relative to one or more of the other segments, providing a more pronounced difference in actual sensitivity from the other segments.

When such a sensor 230 extends among different environments 260a, 260b, 260c,

260d (FIGS. 4, 6 and 7), variations in the apparent sensitivity of the sensor 230 can be substantially reduced compared to a sensor having a longitudinally constant actual sensitivity extending among the environments.

For example, with a sensor as shown in FIG. 7, the actual sensitivity (and a corresponding reference-sensor pair) of the first segment 233 can be selected to correspond with one or more characteristics of the corresponding intended environment 260a (e.g., underground). Thus, the segment 233 can be configured to achieve a first apparent sensitivity when the sensor 230 is exposed to the environment 260a. In a similar fashion, the actual sensitivity of the second segment 235 can be selected to correspond with one or more characteristics of the corresponding intended environment 260b (e.g., a wetland). Accordingly, the segment 235 can be configured (e.g., by selecting a reference-sensor pair configuration) to achieve a second apparent sensitivity when the sensor 230 is installed in the environment 260b. The actual sensitivity of the third segment 237 and the fourth segment 239, respectively, can be selected to correspond with one or more characteristics of the corresponding intended environments 260c (e.g., under water), 260d (e.g., in the air) such that the segments 237, 239 achieve respective third and fourth apparent sensitivities when the sensor 230 is installed. The respective first, second, third and fourth apparent sensitivities can be more closely matched to each other than corresponding portions of a sensor having a longitudinally constant actual sensitivity would exhibit when extending among the different environments 260a, 260b, 260c, 260d. ALTERNATIVE SENSOR CONFIGURATIONS

In some sensor embodiments, such as, for example, the sensor 230 shown in FIG. 4, the actual sensitivity of each respective segment differs from the actual sensitivity of each of the other segments; in other instances, the actual sensitivity of each of two or more respective segments is substantially the same. Also, although the sensor 230 is shown as having four segments 233, 235, 237, 239, alternative sensor embodiments can have more or fewer segments. The number of segments (and respective actual sensitivities) can be selected to correspond to the number and types of environments the sensor is expected to be exposed to in use.

In addition, although segments 233, 235, 237, 239 are shown and described as having six optical conduits circumferentially spaced from each other, other segment configurations are possible. For example, a segment can have more or fewer

longitudinally extending optical conduits than shown in FIGS. 5 A and 5B. Optical conduits can be spaced at other than 60-degrees from each other (even when six optical conduits are present in a given segment). A segment can have more or fewer tight- buffered or loose-tube conduits.

Although "loose-tube fibers" and "tight-buffered fibers" are described, any suitable optical conduit can be used. Also, although optical conduits exhibiting two classes of signal propagation or mechanical characteristics (e.g., "loose-tube fibers" and "tight-buffered fibers") are described, the disclosed principles apply to segments having a group of conduits that exhibit more than two classes of signal propagation characteristics. Such characteristics include, by way of example, birefringence, length, phase, propagation time, polarization and coating type.

For a given segment, each pair of optical conduits selected as the operative reference-sensor pair can provide the segment with a unique actual sensitivity. The range of achievable actual sensitivities can correspond, at least in part, to the physical and optical characteristics and relative locations of each in the pair of conduits, as well as to the overall configuration (e.g., the number of optical conduits in the bundle, the respective location of each conduit within the bundle, whether one or more interstitial spaces of the bundle is filled, and if so, the material used to fill the spaces). For each segment in a multi-segment sensor (e.g., the sensor 230 shown in FIG. 4), a respective pair of optical conduits can be selected as the reference-sensor pair based, at least in part, to correspond with a known, or selected, environmental characteristics (e.g., material properties related to vibration-transmission through an environmental material, such as, for example, soil, water or air).

In some instances, a sensor can have a continually varying actual sensitivity along its length. In other instances, the sensor can have a stepwise or discretely varying actual sensitivity along its length, as with the sensor 230 shown in FIG. 4. In some instances, a sensor having individual segments with respective lengths less than or on the order of a spatial resolution of the sensor can exhibit one or more characteristics of a sensor having a continuously varying sensitivity.

In some instances, a sensor can have only one optical conduit to make up the reference-sensor pair (eg., a Sagnac interferometer or a modalmetric sensor). In other cases, more than two optical conduits may be used to create the sensor. OTHER EMBODIMENTS

Using the principles disclosed herein, those of ordinary skill will appreciate a wide variety of possible embodiments of interferometer systems, particularly those configured to detect a disturbance with a sensor extending among two or more environments. For example, although Michelson and Mach-Zehnder interferometers have been described above in some detail, sensors disclosed herein can be used with a variety of other types of interferometers, such as, for example, overlapping first and second Mach Zehnder interferometers, a Sagnac interferometer, a modalmetric sensor, an optical time domain reflectometer (OTDR), such as, for example, a coherent-OTDR interferometer, a polarimeter, and many other interferometer configurations.

This disclosure makes reference to the accompanying drawings which form a part hereof, wherein like numerals designate like parts throughout. The drawings illustrate specific embodiments, but other embodiments may be formed and structural changes may be made without departing from the intended scope of this disclosure. Directions and references (e.g. , up, down, top, bottom, left, right, rearward, forward, etc.) may be used to facilitate discussion of the drawings but are not intended to be limiting. For example, certain terms may be used such as "up," "down,", "upper," "lower," "horizontal," "vertical," "left," "right," and the like. These terms are used, where applicable, to provide some clarity of description when dealing with relative relationships, particularly with respect to the illustrated embodiments. Such terms are not, however, intended to imply absolute relationships, positions, and/or orientations. For example, with respect to an object, an "upper" surface can become a "lower" surface simply by turning the object over. Nevertheless, it is still the same surface and the object remains the same. As used herein, "and/or" means "and" as well as "and" and "or."

Accordingly, this detailed description shall not be construed in a limiting sense, and following a review of this disclosure, those of ordinary skill in the art will appreciate the wide variety of interferometer systems that can be devised and constructed using the various concepts described herein. Moreover, those of ordinary skill in the art will appreciate that the exemplary embodiments disclosed herein can be adapted to various configurations without departing from the disclosed concepts. Thus, in view of the many possible embodiments to which the disclosed principles can be applied, it should be recognized that the above-described embodiments are only examples and should not be taken as limiting in scope. And, although detailed claims have not been presented here since claims are not a necessary component for a provisional patent application, I reserve the right to claim as my invention all that comes within the scope and spirit of the subject matter disclosed herein, including but not limited to all that comes within the scope and spirit of the following paragraphs.

Claims

I reserve the right to claim, inter alia, the subject matter below:

1. A joint in a fiber-optic cable, the joint comprising:

a first optical fiber having a first signal propagation characteristic;

a second optical fiber having a second signal propagation characteristic;

wherein the first fiber and the second fiber are operatively coupled such that light can pass from one of the fibers to the other of the fibers, wherein an actual sensitivity changes from region adjacent the joint to another.

2. The joint of claim 1, further comprising a third optical fiber operatively coupled to a fourth optical fiber such that light can pass from one of the third or fourth optical fibers to the other of the third or fourth optical fibers.

3. The joint of claim 2, further comprising first and second segments, wherein the first optical fiber and the third optical fiber comprise a reference-sensor pair in the first segment and wherein the second optical fiber and the fourth optical fiber comprise a reference-sensor pair in the second segment, and wherein the first segment has a first actual sensitivity and the second segment has a second actual sensitivity that differs from the first actual sensitivity.

4. The joint of claim 3, wherein the third optical fiber and the fourth optical fiber have

different signal propagation characteristics from each other.

5. The joint of claim 3, wherein the first and third optical fibers have substantially identical signal propagation characteristics.

6. The joint of claim 3, wherein the second and fourth optical fibers have substantially identical signal propagation characteristics.

7. The joint of claim 5, wherein each of the first and third optical fibers comprise respective loose-tube fibers or respective tight-buffered fibers.

8. The joint of claim 1, wherein the first optical fiber comprises loose-tube fibers and the second optical fiber comprises tight-buffered fibers.

9. The joint of claim 1, further comprising an optical-fiber fusion splice or a mechanical coupling operatively coupling the first optical fiber and the second optical fiber.

10. A fiber optic cable comprising: a first portion comprising at least one fiber having a first signal propagation characteristic and at least one fiber having a second signal propagation characteristic; a second portion comprising at least one fiber having the first signal propagation characteristic and at least one fiber having the second signal propagation characteristic; and, an operative coupling between the first portion's at least one fiber having the first signal propagation characteristic and the second portion's at least one fiber having the second signal propagation characteristic.

11. The fiber optic cable of claim 10, wherein the operative coupling comprises a first operative coupling, wherein the fiber optic cable further comprises a second operative coupling between the first portion's at least one fiber having a second signal propagation characteristic and the second portion's at least one fiber having the first signal propagation characteristic.

12. The fiber optic cable of claim 10, wherein each fiber having the first signal propagation characteristic comprises a loose-tube fiber.

13. The fiber optic cable of claim 10, wherein each fiber having the second signal

propagation characteristic comprises a tight-buffered fiber.

14. The fiber optic cable of claim 10, wherein a spacing between the at least one fiber having a first signal propagation characteristic and the at least one fiber having a second signal propagation characteristic in the first portion differs from a spacing between the at least one fiber having a first signal propagation characteristic and the at least one fiber having a second signal propagation characteristic in the second portion.

15. The fiber optic cable of claim 10, wherein each fiber comprises a single-mode fiber.

16. The fiber optic cable of claim 10, wherein the at least one fiber having a first signal propagation characteristic in the first portion comprises a plurality of first loose-tube fibers.

17. The fiber optic cable of claim 10, wherein the at least one fiber having a second signal propagation characteristic in the first portion comprises a plurality of first tight-buffer fibers.

18. The fiber optic cable of claim 10, wherein each of the at least one fiber having a first signal propagation characteristic in the first portion and each of the at least one fiber having the first signal propagation characteristic in the second portion comprises respective first and second pluralities of loose-tube fibers; wherein each of the at least one fiber having a second signal propagation characteristic in the first portion and each of the at least one fiber having the second signal propagation characteristic in the second portion comprises respective first and second pluralities of tight-buffer fibers.

19. The fiber optic cable of claim 18, wherein the operative coupling between the first

portion's at least one fiber having the first signal propagation characteristic and the second portion's at least one fiber having the second signal propagation characteristic comprises a first operative coupling between one of the first loose-tube fibers and one of the second tight-buffer fibers.

20. The fiber optic cable of claim 19, further comprising a second operative coupling

between one of the first loose-tube fibers and one of the second loose-tube fibers.

21. The fiber optic cable of claim 19, further comprising a second operative coupling

between one of the first tight buffer fibers and one of the second loose-tube fibers.

22. The fiber optic cable of claim 19, further comprising a second operative coupling between one of the first tight buffer fibers and one of the second tight-buffer fibers.

23. The fiber optic cable of claim 18, wherein each plurality of loose-tube fibers comprises two loose-tube fibers and wherein each plurality of tight-buffer fibers comprises four tight-buffer fibers.

24. The fiber optic cable of claim 23, wherein each of the fibers defines a longitudinal axis and the cable further comprises an outer sheath defining a generally circular cross-section and a corresponding central longitudinal axis, and wherein each fiber's longitudinal axis is positioned radially outward of the central longitudinal axis.

25. The fiber optic cable of claim 24, wherein each of the loose-tube fibers is positioned about 180-degrees from each other.

26. The fiber optic cable of claim 24, wherein each of the tight-buffer fibers is positioned about 180-degrees from one of the other tight-buffer fibers.

27. An interferometer configured to detect a disturbance, the interferometer comprising: an active portion configured to emit light into an optical sensor, to receive at least one optical signal from the optical sensor, or both; and

an optical sensor operatively coupled with the active portion, wherein the optical sensor comprises a first segment having a first actual sensitivity and a second segment having a second actual sensitivity.

28. The interferometer of claim 27, wherein the first segment is configured to provide a first apparent sensitivity when exposed to a first environment and wherein the second segment is configured to provide a second apparent sensitivity when exposed to a second environment, wherein the first environment differs from the second environment and wherein the first apparent sensitivity and the second apparent sensitivity are substantially the same.

29. The interferometer of claim 27, wherein the active portion and the optical sensor are together configured to form a selected one of the group consisting of: (i) overlapping first and second Mach Zehnder interferometers; (ii) a Sagnac interferometer; (iii) a

modalmetric sensor; (iv) a coherent-OTDR; and (v) a polarimeter.

30. The interferometer of claim 29, wherein the optical sensor is further configured to

convey an optical such that a disturbance to a portion of the sensor tends to modify the optical signal.

31. The interferometer of claim 30, wherein the active portion is further configured to

monitor the respective optical signals and to infer therefrom a location of a disturbance to the sensor.

32. The interferometer of claim 29, wherein the first segment is configured to provide a first apparent sensitivity when exposed to a first environment and wherein the second segment is configured to provide a second apparent sensitivity when exposed to a second environment, wherein the first environment differs from the second environment and wherein the first apparent sensitivity and the second apparent sensitivity are substantially the same.

33. The interferometer of claim 27, wherein the active portion and the optical sensor are together configured to form a hybrid Michelson and Mach Zehnder interferometer.

34. The interferometer of claim 33, wherein the first segment is configured provide a first apparent sensitivity when exposed to a first environment and wherein the second segment is configured to provide a second apparent sensitivity when exposed to a second environment, wherein the first environment differs from the second environment and wherein the first apparent sensitivity and the second apparent sensitivity are substantially the same.

35. The interferometer of claim 27, further comprising one or more additional segments, each having a respective actual sensitivity different from the first actual sensitivity or the second actual sensitivity.

36. The interferometer of claim 35, wherein each of the one or more additional segments is configured to provide a corresponding apparent sensitivity when exposed to a respective corresponding environment, and wherein each corresponding apparent sensitivity is substantially the same as the first apparent sensitivity and the second apparent sensitivity.